Data Sheet

ADP1851

Rev. B | Page 15 of 24

APPLICATIONS INFORMATION

ADIsimPower DESIGN TOOL

The ADP1851 is supported by the ADIsimPower™ design tool

set. ADIsimPower is a collection of tools that produce complete

power designs optimized for a specific design goal. The tools

allow the user to generate a full schematic and bill of materials

and to calculate performance in minutes. ADIsimPower can

optimize designs for cost, area, efficiency, and parts count while

taking into consideration the operating conditions and limitations

of the IC and all real external components. The ADIsimPower

tool can be found at www.analog.com/ADIsimPower, and the

user can request an unpopulated board through the tool.

SETTING THE OUTPUT VOLTAGE

The output voltage is set using a resistive voltage divider from

















FB

FB

OUT

BOT

TOP

V

V

V

R

R

where:

SOFT START

The soft start period is set by an external capacitor between SS

and AGND. The soft start function limits the input inrush current

and prevents output overshoot. When EN is enabled, a current

source of 6.5 μA starts charging the capacitor, and the regulation

voltage is reached when the voltage at SS reaches 0.6 V. The soft

start time is approximated by

SS

SS

C

t

μA

5

.

6

V

6

.

0



The SS pin reaches a final voltage equal to VCCO.

When a controller is disabled, for example, if EN is pulled low or

experiences an overcurrent limit condition, the soft start capacitor

is discharged through an internal 3 kΩ pull-down resistor.

SETTING THE CURRENT LIMIT

The current-limit comparator measures the voltage across the

low-side MOSFET to determine the load current.

between ILIM and SW. The current sense pin, ILIM, sources

nominally 50 μA to this external resistor. This creates an offset

or greater than this offset voltage, the ADP1851 flags a current-

limit event.

μA

50

06

.

1

CS

LPK

ILIM

R

I

R







where:

ACCURATE CURRENT-LIMIT SENSING

temperature range. Accurate current-limit sensing is achieved

by adding a current sense resistor from the source of the low-

side MOSFET to PGND. Make sure that the power rating of the

current sense resistor is adequate for the application. Figure 24

illustrates the implementation of accurate current-limit sensing.

ADP1851

DH

SW

ILIM

DL

Figure 24. Accurate Current-Limit Sensing

INPUT CAPACITOR SELECTION

Use two parallel capacitors placed close to the drain of the high-

side switch MOSFET (one bulk capacitor of sufficiently high

current rating and a 10 μF ceramic decoupling capacitor).

Select the input bulk capacitor based on its ripple current

rating. The minimum input capacitance required for a

particular load is

SW

ESR

O

PP

O

MIN

IN

f

DR

I

V

D

D

I

C

)

(

)

1

(

,











where:

D is the duty cycle.